The present invention relates to an exhaust emission control device for a motor bicycle.
FIG. 5 is a longitudinal sectional view of an exhaust emission control device 100 for use in a motor bicycle and, in particular, a scooter-type motor bicycle. In the exhaust emission control device 100, an exhaust pipe 101 extending from the exhaust port of the associated engine is connected to a muffler body 102.
The inner space of the muffler body 102 is divided into three expansion chambers 105a, 105b and 105c by partitions 103 and 104. The exhaust pipe 101 is inserted through the front section of the muffler body 102 to open into the expansion chamber 105a.
A catalyst 106 is provided in the partition 103 in such a way as to penetrate therethrough, and a connection pipe 107 passes through the partition 104. A discharge pipe 108 for allowing the expansion chamber 105c to communicate with the exterior of the muffler body 102 is provided at the rearmost end of the muffler body 102.
In this exhaust emission control device 100, exhaust gas from the exhaust pipe 101 enters the expansion chamber 105a and passes through the catalyst 106 to flow into the expansion chamber 105b. Then, it flows through the connection pipe 107 to enter the expansion chamber 105c, from which it is discharged to the exterior through the discharge pipe 108. In this process, the harmful components of the exhaust gas, such as hydrocarbon HC and carbon monoxide CO, are removed therefrom by the catalyst 106 and, at the same time, the exhaust gas expands inside the expansion chambers 105a, 105b and 105c to effect a muffling function.
In this exhaust emission control device 100, however, the exhaust pipe 101 opens into the foremost section of the muffler body 102, that is, the exhaust pipe 101 is not sufficiently long. As a result, the engine output characteristics may deteriorate. In particular, the torque during low-speed running tends to be rather weak.
In view of this, an exhaust emission control device 110 as shown in FIG. 6 has been developed. Like the above-described exhaust emission control device 100, this exhaust emission control device 110 is divided into three expansion chambers 114a, 114b and 114c by partitions 112 and 113. An exhaust pipe 115, which is inserted through the front end of a muffler body 111, extends through the partition 112 to open into the rear section of the expansion chamber 114b.
A catalyst 116 is provided in the partition 113 in such a way as to penetrate therethrough. A connection pipe 117 for connecting the expansion chambers 114a and 114c to each other extends through the partitions 112 and 113. Further, a discharge pipe 118 for allowing the expansion chamber 114a to communicate with the exterior of the muffler body 111 extends through the partitions 112 and 113 and the rearmost section of the muffler body 111.
In this exhaust emission control device 110, the exhaust gas which flows through the exhaust pipe 115 first enters the intermediate expansion chamber 114b and then passes through the catalyst 116 to enter the rear expansion chamber 114c. Then, it flows through the connection pipe 117 to enter the front expansion chamber 114a before it is finally discharged to the exterior of the muffler body 111 through the discharge pipe 118.
In the thus constructed exhaust emission control device 110, the length of the portion of the exhaust pipe 115 inside the muffler body 111 is increased, so that a sufficient overall exhaust-pipe length is secured, thereby improving the engine output characteristics.
However, in the above-described structure, the catalyst 116, whose temperature is raised as a result of its reaction with the exhaust gas, is located in the rearmost section of the muffler body 111, with the result that the rear section of the muffler body 111 becomes hot. Since the rear section of the muffler body 111 generally protrudes beyond the periphery of the motor bicycle, it is liable to come into contact with human bodies and other object and cause harm thereto. Thus, to prevent such harm, it is necessary to provide a shielding plate around the muffler body 111, resulting in an increase in the production cost of the motor bicycle.